WAAS Performances

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA)[1] and is specially developed for the civil aviation community.[2] The system, which was declared operational in late 2003,[3] currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.[4] WAAS service area includes CONUS, Alaska, Canada and Mexico.[5] The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.[6]

WAAS Performance

Satellite Based Augmentation Systems (SBAS) performances are usually described in terms of integrity, accuracy, availability, and continuity.
According to the WAAS performance standard,[7] WAAS service area is divided in five coverage zones:

Zone 1: CONUS.

Zone 2: Alaska.

Zone 3: Hawaii.

Zone 4: Puerto Rico and some other Caribbean islands.

Zone 5: US territory excluding zones 1 to 4.

The volume covered includes a region up to 100.000 feet above the surface. For zones 1 to 3, the area extends up to 30 nm outside the land.

The requirements considering the different flight phases and coverage zones are summarised in the next table:[7]

WAAS Navigation Performance Requirements

En Route

Terminal

LNAV

LNAV/VNAV

LPV

LPV 200

TTA

15 s

15 s

10 s

10 s

6.2 s

6.2 s

HAL

2 nm

1 nm

556 m

556 m

40 m

40 m

VAL

N/A

N/A

N/A

50 m

50 m

35 m

Probability of HMI

10-7 per hour

10-7 per hour

10-7 per hour

2 x 10-7 per approach

2 x 10-7 per approach (150 seconds)

2 x 10-7 per approach (150 seconds)

Zone 1 Continuity

1 - 10-5 per hour

1 - 10-5 per hour

1 - 10-5 per hour

1 - 5.5 x 10-5/15 seconds

1 - 8 x 10-6/15 seconds

1 - 8 x 10-6/15 seconds

Horizontal Accuracy (95%)

0.4 nm

0.4 nm

220 m

220 m

16 m

16 m

Vertical Accuracy (95%)

N/A

N/A

N/A

20 m

20 m

4 m

Availability (Zone 1 Coverage)

0.99999 (100%)

0.99999 (100%)

0.99999 (100%)

0.99 (100%)

0.99 (80-100%)

0.99 (40-60%)

Availability (Zone 2 Coverage)

0.999 (100%)

0.999 (100%)

0.999 (100%)

0.95 (75%)

0.95 (75%)

N/A

Availability (Zone 3 Coverage)

0.999 (100%)

0.999 (100%)

0.999 (100%)

N/A

N/A

N/A

Availability (Zone 4 Coverage)

0.999 (100%)

0.999 (100%)

0.999 (100%)

N/A

N/A

N/A

Availability (Zone 5 Coverage)

0.99999 (100%)

0.999 (100%)

0.999 (100%)

N/A

N/A

N/A

In this table, the integrity requirements are given as the probability of HMI, namely Hazardously Misleading Information. In addition to integrity, accuracy, availability and continuity, the table shows the specification for Time To Alert (TTA) and for Horizontal and Vertical Alert Limits (HAL and VAL).

On July 10, 2003, the WAAS system was certified for safety-of-life aviation, covering 95% of the contiguous U.S. and part of Alaska [3][8]. At present, WAAS supports en-route, terminal and approach operations down to a full LPV-200 (CAT-I like Approach Capability) for the CONUS, Mexico and Canada.